[flake8-bugbear] Include certain guaranteed-mutable expressions: tuples, generators, and assignment expressions (B006) (#20024)

## Summary
Resolves #20004

The implementation now supports guaranteed-mutable expressions in the
following cases:
- Tuple literals with mutable elements (supporting deep nesting)
- Generator expressions
- Named expressions (walrus operator) containing mutable components

Preserves original formatting for assignment value:

```python
# Test case
def f5(x=([1, ])):
    print(x)
```
```python
# Fix before
def f5(x=(None)):
    if x is None:
        x = [1]
    print(x)
```
```python
# Fix after 
def f5(x=None):
    if x is None:
        x = ([1, ])
    print(x)
```
The expansion of detected expressions and the new fixes gated behind
previews.

## Test Plan
- Added B006_9.py with a bunch of test cases
- Generated snapshots

---------

Co-authored-by: Igor Drokin <drokinii1017@gmail.com>
Co-authored-by: dylwil3 <dylwil3@gmail.com>

crates/ruff_linter/resources/test/fixtures/flake8_bugbear/B006_9.py

@@ -0,0 +1,26 @@
+def f1(x=([],)):
+    print(x)
+
+
+def f2(x=(x for x in "x")):
+    print(x)
+
+
+def f3(x=((x for x in "x"),)):
+    print(x)
+
+
+def f4(x=(z := [1, ])):
+    print(x)
+
+
+def f5(x=([1, ])):
+    print(x)
+
+
+def w1(x=(1,)):
+    print(x)
+
+
+def w2(x=(z := 3)):
+    print(x)

crates/ruff_linter/src/preview.rs

@@ -245,3 +245,17 @@ pub(crate) const fn is_a003_class_scope_shadowing_expansion_enabled(
 pub(crate) const fn is_refined_submodule_import_match_enabled(settings: &LinterSettings) -> bool {
     settings.preview.is_enabled()
 }
+
+// github.com/astral-sh/ruff/issues/20004
+pub(crate) const fn is_b006_check_guaranteed_mutable_expr_enabled(
+    settings: &LinterSettings,
+) -> bool {
+    settings.preview.is_enabled()
+}
+
+// github.com/astral-sh/ruff/issues/20004
+pub(crate) const fn is_b006_unsafe_fix_preserve_assignment_expr_enabled(
+    settings: &LinterSettings,
+) -> bool {
+    settings.preview.is_enabled()
+}

crates/ruff_linter/src/rules/flake8_bugbear/mod.rs

@@ -47,6 +47,7 @@ mod tests {
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_6.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_7.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_8.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_9.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_B008.py"))]
     #[test_case(Rule::MutableArgumentDefault, Path::new("B006_1.pyi"))]
     #[test_case(Rule::NoExplicitStacklevel, Path::new("B028.py"))]
@@ -83,6 +84,17 @@ mod tests {
     }
 
     #[test_case(Rule::MapWithoutExplicitStrict, Path::new("B912.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_1.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_2.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_3.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_4.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_5.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_6.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_7.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_8.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_9.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_B008.py"))]
+    #[test_case(Rule::MutableArgumentDefault, Path::new("B006_1.pyi"))]
     fn preview_rules(rule_code: Rule, path: &Path) -> Result<()> {
         let snapshot = format!(
             "preview__{}_{}",

crates/ruff_linter/src/rules/flake8_bugbear/rules/mutable_argument_default.rs

@@ -3,9 +3,8 @@ use std::fmt::Write;
 use ruff_macros::{ViolationMetadata, derive_message_formats};
 use ruff_python_ast::helpers::is_docstring_stmt;
 use ruff_python_ast::name::QualifiedName;
-use ruff_python_ast::{self as ast, Expr, Parameter};
+use ruff_python_ast::parenthesize::parenthesized_range;
-use ruff_python_codegen::{Generator, Stylist};
+use ruff_python_ast::{self as ast, Expr, ParameterWithDefault};
-use ruff_python_index::Indexer;
 use ruff_python_semantic::SemanticModel;
 use ruff_python_semantic::analyze::function_type::is_stub;
 use ruff_python_semantic::analyze::typing::{is_immutable_annotation, is_mutable_expr};
@@ -13,8 +12,11 @@ use ruff_python_trivia::{indentation_at_offset, textwrap};
 use ruff_source_file::LineRanges;
 use ruff_text_size::Ranged;
 
-use crate::Locator;
 use crate::checkers::ast::Checker;
+use crate::preview::{
+    is_b006_check_guaranteed_mutable_expr_enabled,
+    is_b006_unsafe_fix_preserve_assignment_expr_enabled,
+};
 use crate::{Edit, Fix, FixAvailability, Violation};
 
 /// ## What it does
@@ -111,8 +113,12 @@ pub(crate) fn mutable_argument_default(checker: &Checker, function_def: &ast::St
             .iter()
             .map(|target| QualifiedName::from_dotted_name(target))
             .collect();
-
+        let is_mut_expr = if is_b006_check_guaranteed_mutable_expr_enabled(checker.settings()) {
-        if is_mutable_expr(default, checker.semantic())
+            is_guaranteed_mutable_expr(default, checker.semantic())
+        } else {
+            is_mutable_expr(default, checker.semantic())
+        };
+        if is_mut_expr
             && !parameter.annotation().is_some_and(|expr| {
                 is_immutable_annotation(expr, checker.semantic(), extend_immutable_calls.as_slice())
             })
@@ -120,35 +126,37 @@ pub(crate) fn mutable_argument_default(checker: &Checker, function_def: &ast::St
             let mut diagnostic = checker.report_diagnostic(MutableArgumentDefault, default.range());
 
             // If the function body is on the same line as the function def, do not fix
-            if let Some(fix) = move_initialization(
+            if let Some(fix) = move_initialization(function_def, parameter, default, checker) {
-                function_def,
-                &parameter.parameter,
-                default,
-                checker.semantic(),
-                checker.locator(),
-                checker.stylist(),
-                checker.indexer(),
-                checker.generator(),
-            ) {
                 diagnostic.set_fix(fix);
             }
         }
     }
 }
 
+/// Returns `true` if the expression is guaranteed to create a mutable object.
+fn is_guaranteed_mutable_expr(expr: &Expr, semantic: &SemanticModel) -> bool {
+    match expr {
+        Expr::Generator(_) => true,
+        Expr::Tuple(ast::ExprTuple { elts, .. }) => {
+            elts.iter().any(|e| is_guaranteed_mutable_expr(e, semantic))
+        }
+        Expr::Named(ast::ExprNamed { value, .. }) => is_guaranteed_mutable_expr(value, semantic),
+        _ => is_mutable_expr(expr, semantic),
+    }
+}
+
 /// Generate a [`Fix`] to move a mutable argument default initialization
 /// into the function body.
-#[expect(clippy::too_many_arguments)]
 fn move_initialization(
     function_def: &ast::StmtFunctionDef,
-    parameter: &Parameter,
+    parameter: &ParameterWithDefault,
     default: &Expr,
-    semantic: &SemanticModel,
+    checker: &Checker,
-    locator: &Locator,
-    stylist: &Stylist,
-    indexer: &Indexer,
-    generator: Generator,
 ) -> Option<Fix> {
+    let indexer = checker.indexer();
+    let locator = checker.locator();
+    let stylist = checker.stylist();
+
     let mut body = function_def.body.iter().peekable();
 
     // Avoid attempting to fix single-line functions.
@@ -157,25 +165,58 @@ fn move_initialization(
         return None;
     }
 
+    let range = match parenthesized_range(
+        default.into(),
+        parameter.into(),
+        checker.comment_ranges(),
+        checker.source(),
+    ) {
+        Some(range) => range,
+        None => default.range(),
+    };
     // Set the default argument value to `None`.
-    let default_edit = Edit::range_replacement("None".to_string(), default.range());
+    let default_edit = Edit::range_replacement("None".to_string(), range);
 
     // If the function is a stub, this is the only necessary edit.
-    if is_stub(function_def, semantic) {
+    if is_stub(function_def, checker.semantic()) {
         return Some(Fix::unsafe_edit(default_edit));
     }
 
     // Add an `if`, to set the argument to its original value if still `None`.
     let mut content = String::new();
-    let _ = write!(&mut content, "if {} is None:", parameter.name());
+    let _ = write!(&mut content, "if {} is None:", parameter.parameter.name());
     content.push_str(stylist.line_ending().as_str());
     content.push_str(stylist.indentation());
+    if is_b006_unsafe_fix_preserve_assignment_expr_enabled(checker.settings()) {
+        let annotation = if let Some(ann) = parameter.annotation() {
+            format!(": {}", locator.slice(ann))
+        } else {
+            String::new()
+        };
+        let _ = write!(
+            &mut content,
+            "{}{} = {}",
+            parameter.parameter.name(),
+            annotation,
+            locator.slice(
+                parenthesized_range(
+                    default.into(),
+                    parameter.into(),
+                    checker.comment_ranges(),
+                    checker.source()
+                )
+                .unwrap_or(default.range())
+            )
+        );
+    } else {
         let _ = write!(
             &mut content,
             "{} = {}",
             parameter.name(),
-        generator.expr(default)
+            checker.generator().expr(default)
         );
+    }
+
     content.push_str(stylist.line_ending().as_str());
 
     // Determine the indentation depth of the function body.

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__B006_B006_9.py.snap

@@ -0,0 +1,22 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_9.py:17:11
+   |
+17 | def f5(x=([1, ])):
+   |           ^^^^^
+18 |     print(x)
+   |
+help: Replace with `None`; initialize within function
+14 |     print(x)
+15 | 
+16 | 
+   - def f5(x=([1, ])):
+17 + def f5(x=None):
+18 +     if x is None:
+19 +         x = [1]
+20 |     print(x)
+21 | 
+22 | 
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_1.py.snap

@@ -0,0 +1,21 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_1.py:3:22
+  |
+1 | # Docstring followed by a newline
+2 |
+3 | def foobar(foor, bar={}):    
+  |                      ^^
+4 |     """
+5 |     """
+  |
+help: Replace with `None`; initialize within function
+1 | # Docstring followed by a newline
+2 | 
+  - def foobar(foor, bar={}):    
+3 + def foobar(foor, bar=None):    
+4 |     """
+5 |     """
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_1.pyi.snap

@@ -0,0 +1,4 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_2.py.snap

@@ -0,0 +1,22 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_2.py:4:22
+  |
+2 | # Regression test for https://github.com/astral-sh/ruff/issues/7155
+3 |
+4 | def foobar(foor, bar={}):    
+  |                      ^^
+5 |     """
+6 |     """                    
+  |
+help: Replace with `None`; initialize within function
+1 | # Docstring followed by whitespace with no newline
+2 | # Regression test for https://github.com/astral-sh/ruff/issues/7155
+3 | 
+  - def foobar(foor, bar={}):    
+4 + def foobar(foor, bar=None):    
+5 |     """
+6 |     """                    
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_3.py.snap

@@ -0,0 +1,20 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_3.py:4:22
+  |
+4 | def foobar(foor, bar={}):    
+  |                      ^^
+5 |     """
+6 |     """
+  |
+help: Replace with `None`; initialize within function
+1 | # Docstring with no newline
+2 | 
+3 | 
+  - def foobar(foor, bar={}):    
+4 + def foobar(foor, bar=None):    
+5 |     """
+6 |     """
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_4.py.snap

@@ -0,0 +1,22 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_4.py:7:26
+  |
+6 | class FormFeedIndent:
+7 |    def __init__(self, a=[]):
+  |                         ^^
+8 |         print(a)
+  |
+help: Replace with `None`; initialize within function
+4  | 
+5  | 
+6  | class FormFeedIndent:
+   -    def __init__(self, a=[]):
+7  +    def __init__(self, a=None):
+8  +         if a is None:
+9  +          a = []
+10 |         print(a)
+11 | 
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_5.py.snap

@@ -0,0 +1,314 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_5.py:5:49
+  |
+5 | def import_module_wrong(value: dict[str, str] = {}):
+  |                                                 ^^
+6 |     import os
+  |
+help: Replace with `None`; initialize within function
+2  | # https://github.com/astral-sh/ruff/issues/7616
+3  | 
+4  | 
+   - def import_module_wrong(value: dict[str, str] = {}):
+5  + def import_module_wrong(value: dict[str, str] = None):
+6  |     import os
+7  +     if value is None:
+8  +         value: dict[str, str] = {}
+9  | 
+10 | 
+11 | def import_module_with_values_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:9:61
+   |
+ 9 | def import_module_with_values_wrong(value: dict[str, str] = {}):
+   |                                                             ^^
+10 |     import os
+   |
+help: Replace with `None`; initialize within function
+6  |     import os
+7  | 
+8  | 
+   - def import_module_with_values_wrong(value: dict[str, str] = {}):
+9  + def import_module_with_values_wrong(value: dict[str, str] = None):
+10 |     import os
+11 | 
+12 +     if value is None:
+13 +         value: dict[str, str] = {}
+14 |     return 2
+15 | 
+16 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:15:50
+   |
+15 | def import_modules_wrong(value: dict[str, str] = {}):
+   |                                                  ^^
+16 |     import os
+17 |     import sys
+   |
+help: Replace with `None`; initialize within function
+12 |     return 2
+13 | 
+14 | 
+   - def import_modules_wrong(value: dict[str, str] = {}):
+15 + def import_modules_wrong(value: dict[str, str] = None):
+16 |     import os
+17 |     import sys
+18 |     import itertools
+19 +     if value is None:
+20 +         value: dict[str, str] = {}
+21 | 
+22 | 
+23 | def from_import_module_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:21:54
+   |
+21 | def from_import_module_wrong(value: dict[str, str] = {}):
+   |                                                      ^^
+22 |     from os import path
+   |
+help: Replace with `None`; initialize within function
+18 |     import itertools
+19 | 
+20 | 
+   - def from_import_module_wrong(value: dict[str, str] = {}):
+21 + def from_import_module_wrong(value: dict[str, str] = None):
+22 |     from os import path
+23 +     if value is None:
+24 +         value: dict[str, str] = {}
+25 | 
+26 | 
+27 | def from_imports_module_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:25:55
+   |
+25 | def from_imports_module_wrong(value: dict[str, str] = {}):
+   |                                                       ^^
+26 |     from os import path
+27 |     from sys import version_info
+   |
+help: Replace with `None`; initialize within function
+22 |     from os import path
+23 | 
+24 | 
+   - def from_imports_module_wrong(value: dict[str, str] = {}):
+25 + def from_imports_module_wrong(value: dict[str, str] = None):
+26 |     from os import path
+27 |     from sys import version_info
+28 +     if value is None:
+29 +         value: dict[str, str] = {}
+30 | 
+31 | 
+32 | def import_and_from_imports_module_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:30:66
+   |
+30 | def import_and_from_imports_module_wrong(value: dict[str, str] = {}):
+   |                                                                  ^^
+31 |     import os
+32 |     from sys import version_info
+   |
+help: Replace with `None`; initialize within function
+27 |     from sys import version_info
+28 | 
+29 | 
+   - def import_and_from_imports_module_wrong(value: dict[str, str] = {}):
+30 + def import_and_from_imports_module_wrong(value: dict[str, str] = None):
+31 |     import os
+32 |     from sys import version_info
+33 +     if value is None:
+34 +         value: dict[str, str] = {}
+35 | 
+36 | 
+37 | def import_docstring_module_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:35:59
+   |
+35 | def import_docstring_module_wrong(value: dict[str, str] = {}):
+   |                                                           ^^
+36 |     """Docstring"""
+37 |     import os
+   |
+help: Replace with `None`; initialize within function
+32 |     from sys import version_info
+33 | 
+34 | 
+   - def import_docstring_module_wrong(value: dict[str, str] = {}):
+35 + def import_docstring_module_wrong(value: dict[str, str] = None):
+36 |     """Docstring"""
+37 |     import os
+38 +     if value is None:
+39 +         value: dict[str, str] = {}
+40 | 
+41 | 
+42 | def import_module_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:40:49
+   |
+40 | def import_module_wrong(value: dict[str, str] = {}):
+   |                                                 ^^
+41 |     """Docstring"""
+42 |     import os; import sys
+   |
+help: Replace with `None`; initialize within function
+37 |     import os
+38 | 
+39 | 
+   - def import_module_wrong(value: dict[str, str] = {}):
+40 + def import_module_wrong(value: dict[str, str] = None):
+41 |     """Docstring"""
+42 |     import os; import sys
+43 +     if value is None:
+44 +         value: dict[str, str] = {}
+45 | 
+46 | 
+47 | def import_module_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:45:49
+   |
+45 | def import_module_wrong(value: dict[str, str] = {}):
+   |                                                 ^^
+46 |     """Docstring"""
+47 |     import os; import sys; x = 1
+   |
+help: Replace with `None`; initialize within function
+42 |     import os; import sys
+43 | 
+44 | 
+   - def import_module_wrong(value: dict[str, str] = {}):
+45 + def import_module_wrong(value: dict[str, str] = None):
+46 |     """Docstring"""
+47 +     if value is None:
+48 +         value: dict[str, str] = {}
+49 |     import os; import sys; x = 1
+50 | 
+51 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:50:49
+   |
+50 | def import_module_wrong(value: dict[str, str] = {}):
+   |                                                 ^^
+51 |     """Docstring"""
+52 |     import os; import sys
+   |
+help: Replace with `None`; initialize within function
+47 |     import os; import sys; x = 1
+48 | 
+49 | 
+   - def import_module_wrong(value: dict[str, str] = {}):
+50 + def import_module_wrong(value: dict[str, str] = None):
+51 |     """Docstring"""
+52 |     import os; import sys
+53 +     if value is None:
+54 +         value: dict[str, str] = {}
+55 | 
+56 | 
+57 | def import_module_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:55:49
+   |
+55 | def import_module_wrong(value: dict[str, str] = {}):
+   |                                                 ^^
+56 |     import os; import sys
+   |
+help: Replace with `None`; initialize within function
+52 |     import os; import sys
+53 | 
+54 | 
+   - def import_module_wrong(value: dict[str, str] = {}):
+55 + def import_module_wrong(value: dict[str, str] = None):
+56 |     import os; import sys
+57 +     if value is None:
+58 +         value: dict[str, str] = {}
+59 | 
+60 | 
+61 | def import_module_wrong(value: dict[str, str] = {}):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:59:49
+   |
+59 | def import_module_wrong(value: dict[str, str] = {}):
+   |                                                 ^^
+60 |     import os; import sys; x = 1
+   |
+help: Replace with `None`; initialize within function
+56 |     import os; import sys
+57 | 
+58 | 
+   - def import_module_wrong(value: dict[str, str] = {}):
+59 + def import_module_wrong(value: dict[str, str] = None):
+60 +     if value is None:
+61 +         value: dict[str, str] = {}
+62 |     import os; import sys; x = 1
+63 | 
+64 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_5.py:63:49
+   |
+63 | def import_module_wrong(value: dict[str, str] = {}):
+   |                                                 ^^
+64 |     import os; import sys
+   |
+help: Replace with `None`; initialize within function
+60 |     import os; import sys; x = 1
+61 | 
+62 | 
+   - def import_module_wrong(value: dict[str, str] = {}):
+63 + def import_module_wrong(value: dict[str, str] = None):
+64 |     import os; import sys
+65 +     if value is None:
+66 +         value: dict[str, str] = {}
+67 | 
+68 | 
+69 | def import_module_wrong(value: dict[str, str] = {}): import os
+note: This is an unsafe fix and may change runtime behavior
+
+B006 Do not use mutable data structures for argument defaults
+  --> B006_5.py:67:49
+   |
+67 | def import_module_wrong(value: dict[str, str] = {}): import os
+   |                                                 ^^
+   |
+help: Replace with `None`; initialize within function
+
+B006 Do not use mutable data structures for argument defaults
+  --> B006_5.py:70:49
+   |
+70 | def import_module_wrong(value: dict[str, str] = {}): import os; import sys
+   |                                                 ^^
+   |
+help: Replace with `None`; initialize within function
+
+B006 Do not use mutable data structures for argument defaults
+  --> B006_5.py:73:49
+   |
+73 | def import_module_wrong(value: dict[str, str] = {}): \
+   |                                                 ^^
+74 |     import os
+   |
+help: Replace with `None`; initialize within function

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_6.py.snap

@@ -0,0 +1,23 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_6.py:4:22
+  |
+2 | # Same as B006_2.py, but import instead of docstring
+3 |
+4 | def foobar(foor, bar={}):    
+  |                      ^^
+5 |     import os                    
+  |
+help: Replace with `None`; initialize within function
+1 | # Import followed by whitespace with no newline
+2 | # Same as B006_2.py, but import instead of docstring
+3 | 
+  - def foobar(foor, bar={}):    
+  -     import os                    
+4 + def foobar(foor, bar=None):    
+5 +     import os                    
+6 +     if bar is None:
+7 +         bar = {}
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_7.py.snap

@@ -0,0 +1,23 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_7.py:4:22
+  |
+2 | # Same as B006_3.py, but import instead of docstring
+3 |
+4 | def foobar(foor, bar={}):    
+  |                      ^^
+5 |     import os
+  |
+help: Replace with `None`; initialize within function
+1 | # Import with no newline
+2 | # Same as B006_3.py, but import instead of docstring
+3 | 
+  - def foobar(foor, bar={}):    
+  -     import os
+4 + def foobar(foor, bar=None):    
+5 +     import os
+6 +     if bar is None:
+7 +         bar = {}
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_8.py.snap

@@ -0,0 +1,92 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_8.py:1:19
+  |
+1 | def foo(a: list = []):
+  |                   ^^
+2 |     raise NotImplementedError("")
+  |
+help: Replace with `None`; initialize within function
+  - def foo(a: list = []):
+1 + def foo(a: list = None):
+2 |     raise NotImplementedError("")
+3 | 
+4 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_8.py:5:19
+  |
+5 | def bar(a: dict = {}):
+  |                   ^^
+6 |     """ This one also has a docstring"""
+7 |     raise NotImplementedError("and has some text in here")
+  |
+help: Replace with `None`; initialize within function
+2 |     raise NotImplementedError("")
+3 | 
+4 | 
+  - def bar(a: dict = {}):
+5 + def bar(a: dict = None):
+6 |     """ This one also has a docstring"""
+7 |     raise NotImplementedError("and has some text in here")
+8 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_8.py:10:19
+   |
+10 | def baz(a: list = []):
+   |                   ^^
+11 |     """This one raises a different exception"""
+12 |     raise IndexError()
+   |
+help: Replace with `None`; initialize within function
+7  |     raise NotImplementedError("and has some text in here")
+8  | 
+9  | 
+   - def baz(a: list = []):
+10 + def baz(a: list = None):
+11 |     """This one raises a different exception"""
+12 +     if a is None:
+13 +         a: list = []
+14 |     raise IndexError()
+15 | 
+16 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_8.py:15:19
+   |
+15 | def qux(a: list = []):
+   |                   ^^
+16 |     raise NotImplementedError
+   |
+help: Replace with `None`; initialize within function
+12 |     raise IndexError()
+13 | 
+14 | 
+   - def qux(a: list = []):
+15 + def qux(a: list = None):
+16 |     raise NotImplementedError
+17 | 
+18 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_8.py:19:20
+   |
+19 | def quux(a: list = []):
+   |                    ^^
+20 |     raise NotImplemented
+   |
+help: Replace with `None`; initialize within function
+16 |     raise NotImplementedError
+17 | 
+18 | 
+   - def quux(a: list = []):
+19 + def quux(a: list = None):
+20 |     raise NotImplemented
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_9.py.snap

@@ -0,0 +1,99 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_9.py:1:10
+  |
+1 | def f1(x=([],)):
+  |          ^^^^^
+2 |     print(x)
+  |
+help: Replace with `None`; initialize within function
+  - def f1(x=([],)):
+1 + def f1(x=None):
+2 +     if x is None:
+3 +         x = ([],)
+4 |     print(x)
+5 | 
+6 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+ --> B006_9.py:5:10
+  |
+5 | def f2(x=(x for x in "x")):
+  |          ^^^^^^^^^^^^^^^^
+6 |     print(x)
+  |
+help: Replace with `None`; initialize within function
+2  |     print(x)
+3  | 
+4  | 
+   - def f2(x=(x for x in "x")):
+5  + def f2(x=None):
+6  +     if x is None:
+7  +         x = (x for x in "x")
+8  |     print(x)
+9  | 
+10 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_9.py:9:10
+   |
+ 9 | def f3(x=((x for x in "x"),)):
+   |          ^^^^^^^^^^^^^^^^^^^
+10 |     print(x)
+   |
+help: Replace with `None`; initialize within function
+6  |     print(x)
+7  | 
+8  | 
+   - def f3(x=((x for x in "x"),)):
+9  + def f3(x=None):
+10 +     if x is None:
+11 +         x = ((x for x in "x"),)
+12 |     print(x)
+13 | 
+14 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_9.py:13:11
+   |
+13 | def f4(x=(z := [1, ])):
+   |           ^^^^^^^^^^
+14 |     print(x)
+   |
+help: Replace with `None`; initialize within function
+10 |     print(x)
+11 | 
+12 | 
+   - def f4(x=(z := [1, ])):
+13 + def f4(x=None):
+14 +     if x is None:
+15 +         x = (z := [1, ])
+16 |     print(x)
+17 | 
+18 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_9.py:17:11
+   |
+17 | def f5(x=([1, ])):
+   |           ^^^^^
+18 |     print(x)
+   |
+help: Replace with `None`; initialize within function
+14 |     print(x)
+15 | 
+16 | 
+   - def f5(x=([1, ])):
+17 + def f5(x=None):
+18 +     if x is None:
+19 +         x = ([1, ])
+20 |     print(x)
+21 | 
+22 | 
+note: This is an unsafe fix and may change runtime behavior

crates/ruff_linter/src/rules/flake8_bugbear/snapshots/ruff_linter__rules__flake8_bugbear__tests__preview__B006_B006_B008.py.snap

@@ -0,0 +1,462 @@
+---
+source: crates/ruff_linter/src/rules/flake8_bugbear/mod.rs
+---
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_B008.py:63:25
+   |
+63 | def this_is_wrong(value=[1, 2, 3]):
+   |                         ^^^^^^^^^
+64 |     ...
+   |
+help: Replace with `None`; initialize within function
+60 | # Flag mutable literals/comprehensions
+61 | 
+62 | 
+   - def this_is_wrong(value=[1, 2, 3]):
+63 + def this_is_wrong(value=None):
+64 |     ...
+65 | 
+66 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_B008.py:67:30
+   |
+67 | def this_is_also_wrong(value={}):
+   |                              ^^
+68 |     ...
+   |
+help: Replace with `None`; initialize within function
+64 |     ...
+65 | 
+66 | 
+   - def this_is_also_wrong(value={}):
+67 + def this_is_also_wrong(value=None):
+68 |     ...
+69 | 
+70 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_B008.py:73:52
+   |
+71 | class Foo:
+72 |     @staticmethod
+73 |     def this_is_also_wrong_and_more_indented(value={}):
+   |                                                    ^^
+74 |         pass
+   |
+help: Replace with `None`; initialize within function
+70 | 
+71 | class Foo:
+72 |     @staticmethod
+   -     def this_is_also_wrong_and_more_indented(value={}):
+73 +     def this_is_also_wrong_and_more_indented(value=None):
+74 |         pass
+75 | 
+76 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_B008.py:77:31
+   |
+77 |   def multiline_arg_wrong(value={
+   |  _______________________________^
+78 | |
+79 | | }):
+   | |_^
+80 |       ...
+   |
+help: Replace with `None`; initialize within function
+74 |         pass
+75 | 
+76 | 
+   - def multiline_arg_wrong(value={
+   - 
+   - }):
+77 + def multiline_arg_wrong(value=None):
+78 |     ...
+79 | 
+80 | def single_line_func_wrong(value = {}): ...
+note: This is an unsafe fix and may change runtime behavior
+
+B006 Do not use mutable data structures for argument defaults
+  --> B006_B008.py:82:36
+   |
+80 |     ...
+81 |
+82 | def single_line_func_wrong(value = {}): ...
+   |                                    ^^
+   |
+help: Replace with `None`; initialize within function
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_B008.py:85:20
+   |
+85 | def and_this(value=set()):
+   |                    ^^^^^
+86 |     ...
+   |
+help: Replace with `None`; initialize within function
+82 | def single_line_func_wrong(value = {}): ...
+83 | 
+84 | 
+   - def and_this(value=set()):
+85 + def and_this(value=None):
+86 |     ...
+87 | 
+88 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_B008.py:89:20
+   |
+89 | def this_too(value=collections.OrderedDict()):
+   |                    ^^^^^^^^^^^^^^^^^^^^^^^^^
+90 |     ...
+   |
+help: Replace with `None`; initialize within function
+86 |     ...
+87 | 
+88 | 
+   - def this_too(value=collections.OrderedDict()):
+89 + def this_too(value=None):
+90 |     ...
+91 | 
+92 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_B008.py:93:32
+   |
+93 | async def async_this_too(value=collections.defaultdict()):
+   |                                ^^^^^^^^^^^^^^^^^^^^^^^^^
+94 |     ...
+   |
+help: Replace with `None`; initialize within function
+90 |     ...
+91 | 
+92 | 
+   - async def async_this_too(value=collections.defaultdict()):
+93 + async def async_this_too(value=None):
+94 |     ...
+95 | 
+96 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+  --> B006_B008.py:97:26
+   |
+97 | def dont_forget_me(value=collections.deque()):
+   |                          ^^^^^^^^^^^^^^^^^^^
+98 |     ...
+   |
+help: Replace with `None`; initialize within function
+94  |     ...
+95  | 
+96  | 
+    - def dont_forget_me(value=collections.deque()):
+97  + def dont_forget_me(value=None):
+98  |     ...
+99  | 
+100 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:102:46
+    |
+101 | # N.B. we're also flagging the function call in the comprehension
+102 | def list_comprehension_also_not_okay(default=[i**2 for i in range(3)]):
+    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^
+103 |     pass
+    |
+help: Replace with `None`; initialize within function
+99  | 
+100 | 
+101 | # N.B. we're also flagging the function call in the comprehension
+    - def list_comprehension_also_not_okay(default=[i**2 for i in range(3)]):
+102 + def list_comprehension_also_not_okay(default=None):
+103 |     pass
+104 | 
+105 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:106:46
+    |
+106 | def dict_comprehension_also_not_okay(default={i: i**2 for i in range(3)}):
+    |                                              ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+107 |     pass
+    |
+help: Replace with `None`; initialize within function
+103 |     pass
+104 | 
+105 | 
+    - def dict_comprehension_also_not_okay(default={i: i**2 for i in range(3)}):
+106 + def dict_comprehension_also_not_okay(default=None):
+107 |     pass
+108 | 
+109 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:110:45
+    |
+110 | def set_comprehension_also_not_okay(default={i**2 for i in range(3)}):
+    |                                             ^^^^^^^^^^^^^^^^^^^^^^^^
+111 |     pass
+    |
+help: Replace with `None`; initialize within function
+107 |     pass
+108 | 
+109 | 
+    - def set_comprehension_also_not_okay(default={i**2 for i in range(3)}):
+110 + def set_comprehension_also_not_okay(default=None):
+111 |     pass
+112 | 
+113 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:114:33
+    |
+114 | def kwonlyargs_mutable(*, value=[]):
+    |                                 ^^
+115 |     ...
+    |
+help: Replace with `None`; initialize within function
+111 |     pass
+112 | 
+113 | 
+    - def kwonlyargs_mutable(*, value=[]):
+114 + def kwonlyargs_mutable(*, value=None):
+115 |     ...
+116 | 
+117 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:239:20
+    |
+237 | # B006 and B008
+238 | # We should handle arbitrary nesting of these B008.
+239 | def nested_combo(a=[float(3), dt.datetime.now()]):
+    |                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+240 |     pass
+    |
+help: Replace with `None`; initialize within function
+236 | 
+237 | # B006 and B008
+238 | # We should handle arbitrary nesting of these B008.
+    - def nested_combo(a=[float(3), dt.datetime.now()]):
+239 + def nested_combo(a=None):
+240 |     pass
+241 | 
+242 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:276:27
+    |
+275 | def mutable_annotations(
+276 |     a: list[int] | None = [],
+    |                           ^^
+277 |     b: Optional[Dict[int, int]] = {},
+278 |     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+    |
+help: Replace with `None`; initialize within function
+273 | 
+274 | 
+275 | def mutable_annotations(
+    -     a: list[int] | None = [],
+276 +     a: list[int] | None = None,
+277 |     b: Optional[Dict[int, int]] = {},
+278 |     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+279 |     d: typing_extensions.Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:277:35
+    |
+275 | def mutable_annotations(
+276 |     a: list[int] | None = [],
+277 |     b: Optional[Dict[int, int]] = {},
+    |                                   ^^
+278 |     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+279 |     d: typing_extensions.Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+    |
+help: Replace with `None`; initialize within function
+274 | 
+275 | def mutable_annotations(
+276 |     a: list[int] | None = [],
+    -     b: Optional[Dict[int, int]] = {},
+277 +     b: Optional[Dict[int, int]] = None,
+278 |     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+279 |     d: typing_extensions.Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+280 | ):
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:278:62
+    |
+276 |     a: list[int] | None = [],
+277 |     b: Optional[Dict[int, int]] = {},
+278 |     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+    |                                                              ^^^^^
+279 |     d: typing_extensions.Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+280 | ):
+    |
+help: Replace with `None`; initialize within function
+275 | def mutable_annotations(
+276 |     a: list[int] | None = [],
+277 |     b: Optional[Dict[int, int]] = {},
+    -     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+278 +     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = None,
+279 |     d: typing_extensions.Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+280 | ):
+281 |     pass
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:279:80
+    |
+277 |     b: Optional[Dict[int, int]] = {},
+278 |     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+279 |     d: typing_extensions.Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+    |                                                                                ^^^^^
+280 | ):
+281 |     pass
+    |
+help: Replace with `None`; initialize within function
+276 |     a: list[int] | None = [],
+277 |     b: Optional[Dict[int, int]] = {},
+278 |     c: Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+    -     d: typing_extensions.Annotated[Union[Set[str], abc.Sized], "annotation"] = set(),
+279 +     d: typing_extensions.Annotated[Union[Set[str], abc.Sized], "annotation"] = None,
+280 | ):
+281 |     pass
+282 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:284:52
+    |
+284 | def single_line_func_wrong(value: dict[str, str] = {}):
+    |                                                    ^^
+285 |     """Docstring"""
+    |
+help: Replace with `None`; initialize within function
+281 |     pass
+282 | 
+283 | 
+    - def single_line_func_wrong(value: dict[str, str] = {}):
+284 + def single_line_func_wrong(value: dict[str, str] = None):
+285 |     """Docstring"""
+286 | 
+287 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:288:52
+    |
+288 | def single_line_func_wrong(value: dict[str, str] = {}):
+    |                                                    ^^
+289 |     """Docstring"""
+290 |     ...
+    |
+help: Replace with `None`; initialize within function
+285 |     """Docstring"""
+286 | 
+287 | 
+    - def single_line_func_wrong(value: dict[str, str] = {}):
+288 + def single_line_func_wrong(value: dict[str, str] = None):
+289 |     """Docstring"""
+290 |     ...
+291 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:293:52
+    |
+293 | def single_line_func_wrong(value: dict[str, str] = {}):
+    |                                                    ^^
+294 |     """Docstring"""; ...
+    |
+help: Replace with `None`; initialize within function
+290 |     ...
+291 | 
+292 | 
+    - def single_line_func_wrong(value: dict[str, str] = {}):
+293 + def single_line_func_wrong(value: dict[str, str] = None):
+294 |     """Docstring"""; ...
+295 | 
+296 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:297:52
+    |
+297 | def single_line_func_wrong(value: dict[str, str] = {}):
+    |                                                    ^^
+298 |     """Docstring"""; \
+299 |         ...
+    |
+help: Replace with `None`; initialize within function
+294 |     """Docstring"""; ...
+295 | 
+296 | 
+    - def single_line_func_wrong(value: dict[str, str] = {}):
+297 + def single_line_func_wrong(value: dict[str, str] = None):
+298 |     """Docstring"""; \
+299 |         ...
+300 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:302:52
+    |
+302 |   def single_line_func_wrong(value: dict[str, str] = {
+    |  ____________________________________________________^
+303 | |     # This is a comment
+304 | | }):
+    | |_^
+305 |       """Docstring"""
+    |
+help: Replace with `None`; initialize within function
+299 |         ...
+300 | 
+301 | 
+    - def single_line_func_wrong(value: dict[str, str] = {
+    -     # This is a comment
+    - }):
+302 + def single_line_func_wrong(value: dict[str, str] = None):
+303 |     """Docstring"""
+304 | 
+305 | 
+note: This is an unsafe fix and may change runtime behavior
+
+B006 Do not use mutable data structures for argument defaults
+   --> B006_B008.py:308:52
+    |
+308 | def single_line_func_wrong(value: dict[str, str] = {}) \
+    |                                                    ^^
+309 |     : \
+310 |     """Docstring"""
+    |
+help: Replace with `None`; initialize within function
+
+B006 [*] Do not use mutable data structures for argument defaults
+   --> B006_B008.py:313:52
+    |
+313 | def single_line_func_wrong(value: dict[str, str] = {}):
+    |                                                    ^^
+314 |     """Docstring without newline"""
+    |
+help: Replace with `None`; initialize within function
+310 |     """Docstring"""
+311 | 
+312 | 
+    - def single_line_func_wrong(value: dict[str, str] = {}):
+313 + def single_line_func_wrong(value: dict[str, str] = None):
+314 |     """Docstring without newline"""
+note: This is an unsafe fix and may change runtime behavior